1. Field of the Invention
The present invention relates to radio receivers for direct sequence spread spectrum communications, and more particularly, to a radio receiver that can detect plural radio signals transmitted at a fixed chipping rate with different length spreading codes.
2. Description of Related Art
Spread spectrum modulation techniques are increasingly desirable for communications, navigation, radar and other applications. In a spread spectrum system, the transmitted signal is spread over a frequency band that is significantly wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference or jamming, and enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced, thereby reducing interference to other radio devices. In view of these significant advantages, spread spectrum communication systems are highly desirable for commercial data transmission.
In one type of spread spectrum communication system, a radio frequency (RF) carrier is modulated by a digital code sequence having a bit rate, or chipping rate, much higher than a clock rate of the information signal. These spread spectrum systems are known as "direct sequence" modulation systems. The RF carrier may be binary or quadrature modulated by one or more data streams such that the data streams have one phase when a code sequence represents a data "one" and a predetermined phase shift (e.g., 180.degree. for binary, and 90.degree. for quadrature) when the code sequence represents a data "zero." These types of modulation are commonly referred to as binary shift key (BPSK) and quadrature shift key (QPSK) modulation, respectively.
It is also known to use a plurality of spread spectrum radio receivers that are coupled together in a wireless local area network (WLAN). A central host processing unit could send information to and receive information from any one of the plurality of remotely disposed receivers. In such a WLAN, the remote receivers may comprise portable units that operate within a defined environment to report information back to the central host processing unit. Each of the remote receivers would communicate with the host processing unit using the same RF carrier frequency and digital code sequence. It should be apparent that such WLAN systems offer increased flexibility over hard-wired systems by enabling operators of the remote receivers substantial freedom of movement through the environment.
The individual radio receivers amplify and filter an RF signal transmitted from the host processing unit to remove the RF carrier and provide a digital information signal that has been modulated by the digital code sequence. The receiver then "de-spreads" the digital signal by use of a digital match filter that is correlated with the digital code sequence to remove the modulation and recover the digital information. Discrete digital bits of the de-spread digital information are then assembled into packets having a predefined format that can be processed subsequently by use of conventional data processing logic systems, such as a microprocessor, digital signal processor, and the like.
In a communication system, energy gain can be defined as the signal-to-jamming ratio. The higher the signal-to-jamming ratio, the more immune the receiver is to jamming interference (or background noise) which increases the effective range of the receiver. For BPSK or QPSK communication systems operating without receiver diversity, the process gain is essentially identical to the energy gain. The process gain in spread spectrum processors may be defined as bandwidth (BW) available for communicating an information signal divided by the data rate (R.sub.b). In decibels (dB), this ratio is defined as follows: EQU PG.sub.dB =10 log.sub.10 (BW/R.sub.b)
According to the preceding equation, for a fixed bandwidth, the processing gain increases as the data rate is decreased. By fixing the chipping rate, the bandwidth is also fixed. A fixed bandwidth is desirable since it allows optimization of the transmit and receive RF circuitry. Therefore, a lower data rate will provide more jamming or noise immunity than a higher data rate, and the processing gain of the receiver is increased approximately 3 dB each time the data rate is halved. Even though the increased jamming immunity is desirable in such WLAN systems, the associated reduction in data rate tends to degrade the rate of data throughput within the overall system. As a result, communication system designers must balance the two desirable attributes of jamming immunity and data throughput in order to provide an acceptable and practical system solution.
Thus, it would be desirable to provide a direct sequence spread spectrum communication system having a fixed chipping rate that achieves the jamming immunity of a low data rate system while also having the data throughput of a high data rate system.